Process for the preparation of fluid bed vinyl acetate catalyst

This application is a continuation of U.S. application Ser. No. 08/720,496 filed Sep. 30, 1996, abandoned, which is a continuation-in-part of U.S. application Ser. No. 08/376,180, filed Jan. 20, 1995, now U.S. Pat. No. 5,591,688, which is a continuation-in-part of U.S. application Ser. No. 08/200,130, filed Feb. 22, 1994, now U.S. Pat. No. 5,466,652, incorporated by reference herein.

The present invention relates to a process for producing a fluid bed palladium-promoted catalyst useful in the production of vinyl acetate from ethylene, acetic acid and oxygen-containing gas. In addition, the present invention relates to a novel fluid bed support and process of using the support for the manufacture of palladium-promoted fluid bed catalyst used in the manufacture of vinyl acetate.

The production of vinyl acetate by reacting ethylene, acetic acid and oxygen together in the gas phase in the presence of a catalyst is known. Typically, the catalysts are in fixed bed form and supported on a porous carrier material such as silica or alumina.

Early examples of these catalysts show that palladium and gold are distributed more or less uniformly throughout the carrier (see, for example, U.S. Pat. Nos. 3,275,680, 3,743,607 and 3,950,400 and Great Britain Patent No. 1,333,449 and South African Patent No. 687,990). Subsequently, it was recognized that this was a disadvantage since it was found that the material on the inner part of the carrier did not contribute to the reaction since the reactants did not significantly diffuse into the carrier. To overcome this problem, new methods of catalyst manufacture were devised with the aim of producing catalyst in which the active components were concentrated on the outer-most shell of the support. For example, Great Britain Patent No. 1,500,167 claims catalyst in which at least ninety percent of the palladium and gold is distributed in that part of the carrier particle which is not more than thirty percent of the particle radius from the surface. In addition, Great Britain Patent No. 1,283,737 teaches that the degree of penetration into the porous carrier can be controlled by pre-treating the porous carrier with an alkaline solution of, for example, sodium carbonate or sodium hydroxide. Another approach which has been found to produce particularly active catalyst is described in U.S. Pat. No. 4,048,096 and other methods of producing shell-impregnated catalyst are disclosed in U.S. Pat. Nos. 4,087,622 and 5,185,308. Each of these patents is primarily concerned with the manufacture of fixed bed catalyst useful for the manufacture of vinyl acetate. However, U.S. Pat. No. 3,950,400 also discloses that the catalyst disclosed therein may be used in a fluid bed reactor. In addition, Great Britain Patent No. 1,266,623 allegedly discloses a fluid bed catalyst for vinyl acetate manufacture which comprises palladium promoted with various alkali, alkaline earth or other metals.

It would be economically beneficial if the manufacture of vinyl acetate could be performed in a fluid bed process as well as a fixed bed process. Some of the typical benefits from a fluid bed process would be that the fluid bed reactor design is simpler than a multi-tubular fixed bed reactor, increased catalyst life is to be expected because no deactivation would take place due to hot spots which are typical of a fixed bed reactor, continuous addition of make-up catalyst can maintain peak performance and virtually eliminate catalyst change-outs, and higher production rates can be expected because substantially higher oxygen levels may be safely fed into the reactor without producing a flammable mixture.

Until the discovery of the process of the present invention, the preparation of palladium-promoted catalyst in fluid bed form has not led to catalyst having the necessary properties leading to a viable economical fluid bed process for the manufacture of vinyl acetate. The process of the present invention overcomes the problems associated with the prior art resulting in a catalyst giving high performance and adequate attrition resistance so that it may be used in the manufacture of vinyl acetate.

It is the primary object of the present invention to provide a process for the manufacture of a fluid bed palladium-metal-promoted alkali metal catalyst useful in the manufacture of vinyl acetate.

It is another object of the present invention to provide a novel support for use in the production of a fluid bed palladium-metal-alkali-metal-promoted catalyst useful in the fluid bed manufacture of vinyl acetate.

It is still another object of the present invention to provide a novel process for the production of a support useful in the manufacture of vinyl acetate catalyst.

Additional objects and advantages of the invention will be set forth in part in the description which follows and, in part, will be obvious from the description or may be learned by practice of the invention. The objects and advantages of the invention may be realized and attained by means of the instrumentalities and combinations particularly pointed out in the appended claims.

To achieve the foregoing objects of the present invention, the process of manufacturing a fluid bed vinyl acetate catalyst characterized by the following formula comprising Pd-M-A wherein M equals barium, cadmium, gold, lanthanum, niobium, cerium, zinc, lead, calcium, strontium, antimony, or mixtures thereof; and A equals at least one alkali metal or mixture thereof comprises impregnating a pre-formed microspheroidal support wherein at least 50% of the particles have a particle size diameter selected to be below 105 microns with a solution comprising a metal salt of the palladium, M and at least one alkali metal and drying the impregnated catalyst. The substantially inert particulate support typically comprises microspheroidal particles selected from the group consisting of alumina, silica, titania, zirconia, and/or mixtures thereof.

In another embodiment of the present invention the process is performed using an aqueous solution free or substantially free of any organic solvent.

In a preferred embodiment of the present invention the metal salt of the alkali metal is separately impregnated onto the support, preferably subsequent to the impregnation of the solution comprising the salts of palladium and M element onto the support material.

In another embodiment of the present invention, the impregnated support may be subjected to known reduction procedures (e.g., heating under reducing conditions) to form a deposit of palladium and M on the surface of the support. The reduction can take place either before or after the deposition of the alkali metal solution.

In a still further preferred embodiment of the present invention the catalyst is dried at a temperature below 80° C., preferably between about 60° to 70° C.

In another preferred embodiment of the present invention the particle size(particle diameter) of the substantially inert support material is selected such that at least 50% of the particles are below about 105 microns. Preferably, at least 75% of the particles are below 105 microns, especially preferred being at least 85% below 105 microns. Finally the preferred support is substantially free of sodium.

In another embodiment of the present invention, the support for the manufacture of the vinyl acetate catalyst comprises a mixture of substantially inert microspheroidal particles having a pore volume of between 0.2 to 0.7 cc/g and/or a surface area of between 50 to 200 m 2 /g and at least 50% of said particle are less than 105 microns.

In a preferred aspect of this embodiment of the present invention, at least 75% of the particles are below 105 microns, especially preferred being at least 85% below 105 microns.

In another embodiment of the present invention, the support for the manufacture of the vinyl acetate catalyst comprises microspheroidal inert particles, preferably silica, zirconia, alumina, titania or mixtures thereof wherein said particles have a pore volume of between 0.2 to 0.7 cc/g and/or a surface area of between 50 to 200 m 2 /g and are obtained from a mixture of less than 100% to 20% inert support sol and greater than zero percent to 80% dried inert particles.

In a preferred embodiment of this aspect of the present invention, the pore volume of the inert particles is between 0.3 to 0.65 cc/g, especially preferred being 0.4 cc to 0.55 cc/g.

In a further preferred embodiment of this aspect of the present invention, the surface area is between 50 to 150 m/g, especially preferred being 60 to 125 m 2 /g.

In a further aspect of this embodiment of the present invention, the silica microspheroidal support material is manufactured by mixing between 20% to less than 100% silica sol with 80% to greater than zero percent silica such as Aerosil®, spray drying said mixture at an elevated temperature of between 115° to 280° C., preferably 130° to 240° C., and calcining said spray dried particles preferably at a temperature of between 550° to 700° C., preferably between 630° to 660° to form the support material.

Typically, the weight percent of the palladium, M and alkaline in the catalyst of the present invention are: 0.1 to 5.0 wt % palladium, preferably 0.2 to 4.0 wt %; greater than 0 to 10 wt % alkali metal, preferably 0.1 to 8.0 wt %, most preferably 0.1 to 5.0 wt %; greater than 0 to about 5.0 wt % M, preferably 0.1 to about 4.0 wt %.

The catalysts of the present invention may be used in a fluid bed reactor for the reaction of ethylene with oxygen to produce vinyl acetate. The reaction temperature is suitably 1000 to 250° C., preferably 135° to 190° C. The reaction pressure is suitably 50 to 200 psig, preferably 75 to 150 psig.

Reference will now be made in detail to the present preferred embodiment of the invention of which the following examples are set forth for illustrative purposes only.

Reactor Testing For Examples 1 to 20

The catalysts were tested in a bench scale fluid bed reactor with a maximum catalyst capacity of 40 cc. Thirty cc of catalyst or catalyst-plus-diluent was the typical volume of solid loaded into the reactor. In general, sufficient catalyst was used such that the reactor contained 0.093 g of palladium metal with each catalyst evaluation. A total of 30 cc volume was obtained by mixing sufficient inert microspheroidal silica with the active catalyst prior to reactor testing. The reactor was equipped with two feed inlets. For some of the experiments of this study, ethylene, acetic acid, and oxygen all entered the reactor through the lower inlet and nitrogen only was fed through the central inlet. In other tests, additional oxygen was fed through the central feed inlet. This central inlet was located 2.5″ above the lower feed inlet.

The reactor pressure was controlled at 115 psig and all lines leading to and from the reactor were heat traced and maintained at 150° to 155° C. order to prevent condensation of liquid feeds or products. Typical temperatures for the fluid bed reactor can vary from 100° to 250° C., preferably 135° to 190° C.

The gaseous reactor effluent was analyzed on-line using a Hewlett Packard Model 5890 gas chromatograph equipped with both TCD and FID detectors. Oxygen, nitrogen, ethylene and carbon dioxide were separated on a 13× mole sieve column parallel with 10% carbowax 20M on 80/100 Chromosorb WAW and 23% SP2700 on 80/100 Chromosorb PAW, and quantitated with the TCD. Vinyl acetate and acetic acid were separated on a 4% carbowax 20M on 80/120 carbopack column and quantitated with the FID.

Support Preparation

Two types of preformed microspheroidal silica were prepared and utilized as supports in the practice of the present invention. Prior to use, all supports were sieved and a specific particle size distribution of the support was used in all catalyst preparations:

It should be understood the particle size distribution recited above is not intended to be limiting and that variations in this distribution are contemplated depending upon reactor size and operating conditions.

Support 1

Support 1 was prepared by spray drying a mixture of Nalco (Nalco Chemical Company) silica sol 1060 and DeGussa Aerosil® (DeGussa Chemical Company) silica. In the dried support, 80% of the silica came from the sol and 20% of the silica came from the Aerosil®. The spray dried microspheres were calcined in air at 640° C. for 4 hours.

Aerosil® silica is the trade name of Degussa's fumed silica. This material has high surface area (˜200 m 2 /g), essentially no micropores, uniform particle size distribution in the nm-range (1×10 9 meter), and is free of sodium. Fumed silica having properties comparable to Aerosil® may be produced by other companies and may be used in the place of Aerosil® in the preparation of Support 1.

Nalco silica sol 1060 is particularly advantageous for use in our application because of large mean particle size of the silica particles in the sol, 60 millimicrons. These larger silica particles pack less efficiently than smaller sol particles (˜30 millimicrons as in Nalco 2327) and yield a final support higher in pore volume in the mesopore region and lower in micropore volume. Other silica sols which have a similarly large (˜40 to 80 millimicrons) mean particle size of the silica may be utilized in the place of the 1060 silica sol in the preparation of Support 1.

Support 2

A series of microspheroidal supports (Supports 2A-2D) containing KA-160 (Sud Chemie) were prepared as follows:

Support 2A: 75% SiO 2 from KA-160 with 25% SiO 2 from Sol

750 g of KA-160 was ground to pass through a 35 mesh screen and washed to remove any soluble impurities, such as chloride ions. This solid silica was then mixed with 694.4 g of Snotex-N-30 (Nissan Chemical) (36 wt % solids) silica sol and 556 g distilled water. This mixture was milled overnight in a jar mill. The smooth slurry was then spray dried to form microspheroidal particles suitable for use in a fluid bed reactor. The microspheroidal support was then calcined at 640° C. in air for 4 hours.

The role of the KA-160 support is to provide much of the pore structure within the microspheroidal particle. The fixed bed support, KA-160, is produced by Sud Chemie and has properties which are advantageous for use in vinyl acetate catalyst preparation. Moderate surface area (160 m 2 /g), little or no microporosity, and substantial porosity (˜0.57 cc/g) in the mesopore region are advantageous properties of KA-160. Alternative fixed bed catalyst supports are available with surface area and pore volume properties similar to KA-160 (little or no micropores, mesopore volume of ˜1.5-0.25 cc/g, and surface area 80-200 m 2 /g). These supports may be utilized in the place of KA-160 in the preparation of Support 2.

Support 2B: 65% SiO 2 from KA-160 with 35% SiO 2 from Sol

This support was prepared in the same manner as Support 2A except that 227.5 g of KA-160, 408.3 g of Snotex-N-30 (30 wt % solids) and 64 g of distilled water were used.

Support 2C: 50% SiO 2 from KA-160 with 50% SiO 2 from Sol

This support was prepared in the same manner as Support 2A except that 175 g of KA-160 and 583.3 g of Snotex-N-30 (30 wt % solids) were used.

Support 2D: 75% SiO 2 from KA-160 with 25% SiO 2 from Sol

This support was prepared in the same manner as Support 2A except that 262 g of KA-160, 219 g of Nalco 2327 (40 wt % solids) (Nalco Chemicals Company) and 219 g of distilled water were used.

Each type of microspheroidal silicas prepared above may be used advantageously in the preparation of fluid bed vinyl acetate monomer catalyst according to the process of the present invention. For use in the manufacture of fluid bed catalysts via impregnation with active metals, these supports provided unexpected superior physical properties for the vinyl acetate catalysts of the present invention compared to any readily available supports. Selected analytical data on all supports are included in Table 1 below.

Catalyst Preparation

The general method utilized in the preparation is summarized below.

Typically, the microspheroidal support is impregnated with a solution (or solutions) of the active metals using the incipient wetness technique. Compounds of the active metals, palladium, M element (e.g. gold) and potassium acetate, may be dissolved in the appropriate ratios in a suitable solvent, then impregnated upon the microspheroidal support. In general, it is desirable if all of the active metals to be used in a catalyst preparation are dissolved in a single portion of solvent which is of the volume just adequate to fill the pore volume of the support. In some instances a desired promoter may not be soluble in the same solvent as the other metal compounds to be used. In this case a solution containing some of the metal components may be impregnated upon the support, followed by impregnating a second solution containing the remaining components. Solvents which are useful include water and volatile organic solvents such as: carboxylic acids with four carbons or less, alcohols, ethers, esters, and aromatics. After the wet catalyst is dried, it may be used for the production of vinyl acetate or it may first be reduced by means known to those skilled in the art.

In general, when acetic acid is present and the catalyst is heated at an elevated temperature (˜100° C.) the catalyst darkens to black and becomes inactive. Additionally, when a solution of palladium acetate (with or without other metal acetates) is heated to too high a temperature or for too long, the solution changes color from the original red-orange to a greenish color and a black precipitate forms. In general, 60° C. is a safe temperature to work at, but up to ˜80° C. has been used for brief periods of time, to dissolve the palladium acetate.